Quantum efficiency for fluorescence

In an effort to assess the importance of this process, Clark and Noxon (58) searched for fluorescent emissions in the wavelength interval 100 to 800 nm upon excitation of CO2 with radiation in the 150 to 170 nm range. Their failure to detect any emissions led them to place a limit of 0.1% upon the quantum efficiency for fluorescence at a CO2 pressure of 0.1 torr. Thus it appears that photodissociation in this wavelength interval occurs as a single-step mechanism with near unit efficiency. 

State lifetimes and modes of energy transfer within the structure. Examples of this are photoluminescence of ZnS nanoparticles studied by Wu et al. (1994), and Mn doped ZnS nanoparticles by Bhargava et al. (1994). In the latter study, the doped nanocrystals were found to have higher quantum efficiency for fluorescence emission than bulk material, and a substantially smaller excited state lifetime. In the case of environmental nanoparticles of iron and manganese oxides, photoluminescence due to any activator dopant would be quenched by magnetic coupling and lattice vibrations. This reduces the utility of photoluminescence studies to excited state lifetimes due to particle-dopant coupling of various types. The fluorescence of uranyl ion sorbed onto iron oxides has been studied in this way, but not as a function of particle size. 

Several general characteristics of photosensitizers affect their efficacy as PDT agents photophysical, photochemical and pharmacological. The photophysical/ photochemical properties include the absorption (extinction spectrum) in vivo, the quantum efficiency for generating singlet oxygen (or other active photoproducts), the photobleaching rate and the quantum efficiency for fluorescence. The characteristics of particular photosensitizers and the relationship to their molecular structure are discussed in other chapters, as are the tissue uptake and clearance and microlocalization properties. Here, we will focus on the methods, primarily optical, that may be used to measure some of these characteristics in vivo. 

Note The quantum efficiency for fluorescence is indicated in parentheses. A (P) indicates useful phosphorescence properties. 

The quantiun yield, or quantum efficiency, for fluorescence or phosphorescence is simply the ratio of the number of molecules that luminesce to the total number of excited molecules. For a highly fluorescent molecule such as fluorescein, the quantum efSciency approaches unity under some conditions. Chemical species that do not fluoresce appreciably have eflicien-cies that approach zero. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

Griseofulvin exhibits both fluorescence and luminescence. A report by Neely et al., (7) gives corrected fluorescence excitation (max. 295 nm) and emission (max. 420 nm) spectra, values for quantum efficiency of fluorescence (0.108) calculated fluorescence lifetime (0.663 nsec) and phosphorescence decay time (0.11 sec.). The fluorescence excitation and emission spectra are given in Figure 7. 

In electroluminescent applications, electrons and holes are injected from opposite electrodes into the conjugated polymers to form excitons. Due to the spin symmetry, only the antisymmetric excitons known as singlets could induce fluorescent emission. The spin-symmetric excitons known as triplets could not decay radiatively to the ground state in most organic molecules [65], Spin statistics predicts that the maximum internal quantum efficiency for EL cannot exceed 25% of the PL efficiency, since the ratio of triplets to singlets is 3 1. This was confirmed by the performance data obtained from OLEDs made with fluorescent organic... 

Cr + ions in aluminum oxide (the ruby laser) show a sharp emission (the so-called Ri emission line) at 694.3 nm. To a good approximation, the shape of this emission is Lorentzian, with Av = 330 GHz at room temperature, (a) Provided that the measured peak transition cross section is c = 2.5 x 10 ° cm and the refractive index is = 1.76, use the formula demonstrated in the previous exercise to estimate the radiative lifetime, (b) Since the measured room temperature fluorescence lifetime is 3 ms, determine the quantum efficiency for this laser material. 

For aromatic hydrocarbons, the quantum efficiencies of fluorescence and phosphorescence in low temperature glasses (Appendix H) such as EPA (ether isopentane ethyl alcohol in the ratio 2 2 5) add up to unity. This suggests that direct nonradiative decay from Si —> S0 is of very low probability. All the nonradiative paths to the ground state are coupled via the triplet state. The sequence of transfers is ... 

Using the charge-balance factory y, the efficiency of production for singlet excitons Tjr, and the quantum efficiency of fluorescence < />/, the internal quantum efficiency... 

The microscopic behaviour between the ions in dilute systems results from multipolar interaction. On the other hand in the rate equations which are used for measurement of macroscopic data such as quantum efficiencies of fluorescence, the multipole questions are absent. The macroscopic treatment of energy transfer was performed recently independently by Fong and Diestler (5) and Grant ( >) who conclude that the concentration... 

No fluorescence spectrum was recorded for rutin solution alone probably because of its low fluorescence quantum efficiency. The fluorescence spectrum recorded for rutin-CTMAB mixture had one strong fluorescence peak (Xex=425.6 nm, Lem=537.9 nm), which indicated the presence of CTMAB surfactant micelles could enhance the fluorescence quantum efficiency of rutin. The fluorescence spectrum of the reaction mixture also had one strong fluorescence peak (Lex=461.3 nm, Lem=539.0 nm), which suggested that the reaction product of rutin was a strongly fluorescent compound. 

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